Offset printing techniques have found considerable success in the newspaper printing industry principally as a result of the simple plate making and changing required, relatively low cost and consistency of printing quality.
The lithographic plate on the plate cylinder in these systems is frequently an engraved drum formed by photographically produced areas that attract oil-based ink and other areas that attract water.
These lithographic offset presses employ clusters of rollers for transfering ink and water, usually in separate clusters, from ink and water supply fountains to the plate cylinder. The last rollers in the cluster are usually rubber or another elastomer that directly engage the plate cylinder and are referred to as form rollers.
In order to evenly distribute ink and water on the plate cylinder it has been found necessary that the number of rollers in each cluster, or train, be quite high and thus significantly contribute to the cost as well as the maintenance of the press.
In the newspaper printing art, where paginated sheets are arranged in columnar fashion, usually nine, the ink demand in each column varies depending upon the amount and arrangement of text, and even more particularly because of the quantity of pictorial images in any column.
This variation in columnar demand at the plate cylinder has in the past been satisfied with separately controllable ink-injecting or metering systems for each column, commonly referred to as "keys". After each new plate change, and frequently more often, the pressman must adjust each of these keys individually to satisfy the ink demand in each column. This is not only a time-consuming task for the pressman that increases printing costs, but frequently more importantly delays newspaper publication. Furthermore, these ink-injecting and metering systems are extremely complex mechanically with a single nine-column injector system frequently having literally hundreds of parts.
There have in the very recent past been attempts to eliminate the need for individual columnar ink-injection and metering systems in the offset printing of paginated newspapers, and while some have achieved a degree of success, they have done so with significant trade-offs. One of these so-called "keyless" systems has been referred to as the "anilox" system and includes the provision of an ink supply roller in the ink train that essentially is a metallic drum with shallow uniform recesses in its exterior surface. This ink-supply drum is coated totally and evenly with ink from the ink fountain and after coating a doctor blade directly engaging the exterior surface of the drum, scrapes all of the ink from the drum save the ink in the shallow recesses. In effect, the "anilox" supply drum acts as a metering drum and while it accomplishes this objective in an accurate fashion, it does have several significant disadvantages. Firstly, the quantity of ink transferred by this supply drum is fixed by the depth, size and number of shallow recesses on the drum surface. Therefore, if the demand of the plate cylinder for ink exceeds the capability of the metering drum, a new metering drum needs to be provided. Another disadvantage is because the doctor blade has direct contact with the metering surface, it experiences considerable wear and needs to be replaced very frequently to avoid streaking on the plate cylinder which results in a streaking image on the newsprint itself.
Furthermore, because the "anilox" metering drum has the capability of tranferring only ink and not water, it must be positioned in the ink train and thus increases the length--and therefore the cost--of the ink train.
There have also in the past been attempts to reduce the length of the ink and water trains through the provision of a combined ink and water proportioning drum. It should be noted at the outset in this discussion, however, that such attempts have been totally apart from the above-described work in attempting to eliminate the requirement for columnar ink injection systems. One such ink train length reduction attampt is illustrated in the Warner U.S. Pat. No. 4,287,827 and includes the provision of a lithographic cylinder in the ink and water supply trains upstream from the plate cylinder. Warner provides a combined water and ink transfer drum that has a plurality of "ink loving" and "water loving" areas on its surface defined by a chromium drum with etched copper areas on its surface. Warner attempted to reduce the length of the ink and water supply trains employing this combined water and ink proportioning drum, but experience at least thus far indicates that the use of this proportioning drum does not significantly, if at all, reduce the required length of the ink and water supply trains.
It is the primary object of the present invention to ameliorate the problems noted above in offset printing systems for the newspaper industry that have heretofore required a plurality of individual columnar metering systems.